Cycloaddition reactions enamine-enal

There is evidence strongly supporting the two-step mechanism. For example, the zwitterionic intermediate has been trapped, and, furthermore, cis- and tranj -isomeric electrophilic olefins give the same product The reaction has been applied to many different conjugated systems and its regio- and stereoselectivity has been extensively studied . Some examples of this heterocycloaddition include the preparation of 1,4-dihydropyran derivative 331, 332 and 333 by intermolecular reaction. In another example 336 was obtained by intramolecular enamine-enal cycloaddition from... 

In 2011 the same research group described another efficient application of dicarbo Q lic acids 63 in the asymmetric inverse-electron-demand 1,3-dipolar cycloaddition (lED 1,3-DC) of C//-cyclic azomethine imines with t-butyl vinyl ether or vinylogous aza-enamines (synthesized from enals) (Scheme 24.23). This latter reaction, carried out without exclusion of moisture and air, gave cycloadducts regioisomeric to the products observed in the normal-electron-demand 1,3-dipolar cycloaddition (NED 1,3-DC) catalysed by Ti/binolate starting from the enals and for this reason the authors introduced the concept of lED umpolung 1,3-DC. 

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